The present invention relates to an improvement in the process and apparatus for making diatomic halogen gas and alkali metal hydroxide by electrolysis of an aqueous alkali metal halide solution employing a bank of two or more cationic permselective membrane cells.
Cationic permselective membrane cells for electrolysis of aqueous alkali metal halide solution to form alkali metal hydroxide and diatomic halide gas are comprised of a housing; an anode and a cathode located within the housing; a cationic permselective membrane separating the anode and the cathode and dividing the housing into an anode compartment and a cathode compartment. In operation, an aqueous alkali metal halide solution is fed to the anode compartment, and water or aqueous alkali metal hydroxide solution is fed to the cathode compartment. A direct electric current is made to flow from the cathode to the anode. It is the primary function of the cationic permselective membrane to permit passage only of positively charged alkali metal ions from the anode compartment to the cathode compartment; negatively charged ions are substantially inhibited from passing through the membrane, as a consequence of the nature of the membrane.
In operation of the cell, as current flows from the cathode to the anode, diatomic halogen gas is formed on the surface of the anode: EQU 2Hal.sup.- .fwdarw. Hal.sub.2 + 2e.sup.-
Alkali metal cations pass through the membrane from the anode compartment to the cathode compartment under the influence of the electrical field, because the positively charged alkali metal cations are attracted by the negatively charged cathode and are repelled by the positively charged anode. A certain amount of endosmotic water is transferred through the membrane from the anode compartment to the cathode compartment. On the surface of the cathode, water is decomposed into hydrogen gas and hydroxide ions: EQU 2e.sup.- + 2H.sub.2 O .fwdarw. H.sub.2 + 2OH.sup.- EQU gas bubbles forming on the surface of the electrodes - diatomic halogen gas at the anode, hydrogen gas at the cathode - tend to inhibit current flow, raising overall electrical resistance of the cell. Also, depletion of the alkali metal halide, resulting in dilution of the brine (anolyte) in the anode compartment, further increases the resistance within the cell. To minimize cell resistance caused by build-up of gas bubbles on the surface of the electrodes and by dilute electrolyte, it is common practice to circulate the aqueous alkali metal halide solution through the anode compartment at a relatively high rate, so that only between about 5 and 30 percent of the alkali metal halide contained in the brine fed to the anode compartment is actually decomposed as the brine is passed therethrough. High rate of recirculation of the anolyte feed results in turbulence of the anolyte within the anode compartment, causing the halogen gas bubbles to disengage more readily from the surface of the anode, thereby lowering the electrical resistance of the cell.
However, there is only relatively limited mass flow through the membrane, relative to the anolyte feed flow rate, as a consequence of migration of alkali metal cations and endosmotic water through the membrane. In order to produce alkali metal hydroxide solution of the proper concentration, it is usual practice to feed water directly to the cathode compartment to supplement the endosmotic water flow associated with the alkali metal cations. Feeding water to the cathode compartment provides means for controlling the alkali metal hydroxide concentration of the catholyte withdrawn from the cell at any desired concentration, within the range of up to about 50 percent by weight of alkali metal hydroxide, based on the combined weight of the water and alkali metal hydroxide.
In the process for electrolysis of aqueous alkali metal halide solutions, high current efficiency is a much sought goal because efficient utilization of energy results in lower product costs. In cationic permselective membrane cells, current efficiency is reduced by back-migration of hydroxide ions through the membrane from the cathode compartment to the anode compartment. The extent of this back-migration depends on many factors, such as composition and treatment of the membrane; operating conditions within the cell such as temperatures, current densities, spacing of electrodes in relation to the membrane; as well as the ion concentration within the anolyte and catholyte. Back-migration tends to be very much increased with increasing hydroxide ion concentration in the catholyte.
Back-migration occurs because the negatively charged hydroxide ions present in the catholyte are attracted to the positively charged anode and are repelled by the negatively charged cathode. Under the influence of the electric potential field in the cell, the hydroxide ions have a relatively strong tendency to back-migrate. Conversely, however, halide ions have little tendency to migrate from the anode compartment to the cathode compartment since, under the influence of the electric potential field, the negatively charged halide ions will move in the direction of the positively charged anode, and are actually repelled by the negatively charged cathode. This factor, combined with the cationic permselective properties of the membrane, results in very little migration of halide ion through the membrane. Consequently, alkali metal hydroxide solutions obtained in permselective membrane cells are usually low in halide ions.
Back-migration of hydroxide ions results in reaction of hydroxide ions at the anode surface to produce oxygen gas, according to the following equation: EQU 4OH.sup.- .fwdarw. O.sub.2 + 2H.sub.2 O + 4e.sup.-
A side reaction that results from back-migration of hydroxide ions in chlorine/alkali metal hydroxide electrolytic membrane cells is the reaction of hydroxide ions with dissolved chlorine gas to form hypochlorous acid. The hypochlorous acid thus formed is relatively unstable, and reacts further to form chlorates. These reactions are illustrated by the following equations: EQU Cl.sub.2 + OH.sup.- .fwdarw. Cl.sup.- + HOCl EQU HOCl .fwdarw. H.sup.+ + OCl.sup.- EQU 2HOCl + OCl.sup.- .fwdarw. ClO.sup.-.sub.3 + 2Cl.sup.- + 2H.sup.+
as a result of these side reactions electrical current is consumed in the formation of products other than diatomic halogen gas and alkali metal hydroxide.
It is an object of the present invention to provide an improvement in the operation of cationic permselective membrane cells for electrolysis of aqueous alkali metal hydroxide solution to form alkali metal hydroxide and diatomic halide gas offering improved current efficiency, and to provide apparatus for practicing such method.
It is a further object of the present invention to provide an improvement in the apparatus for and operation of cationic permselective membrane cells for electrolysis of aqueous alkali metal halide solutions to permit production of alkali metal hydroxide solution of high concentration at improved current efficiency.